The Geneva talks have failed to build up any speed despite eight rounds of negotiations

(Adds quotes from opposition negotiator)

By Tom Miles

GENEVA, Dec 7 (Reuters) - The mediator of U.N.-led Syrian peace talks in Geneva will assess next week whether either side is trying to sabotage the process, he said on Thursday, after President Bashar al-Assad's negotiators said they would turn up five days late.

"We shall assess the behaviour of both sides, government and opposition, in Geneva," U.N. envoy Staffan de Mistura said. "And based on that we will then decide how this... can be a building up or not, or a sabotage of Geneva."

If either side were seen to be sabotaging the process it could have "a very bad impact on any other political attempt to have processes elsewhere," he said.

He said the Geneva rounds of talks were the only peace process backed by the U.N. Security Council, although there were many other initiatives being planned.

He did not elaborate, but Russian President Vladimir Putin, who is seeking re-election next year, has suggested holding a "Syrian Congress" in the Russian city of Sochi early in 2018.

Diplomats see Putin's plan as a bid to draw a line under the war after seven years of fighting and to celebrate Russia's role as the power that tipped the balance of the war and became the key player in the peace process.

Opposition negotiator Basma Kodmani told reporters the government side had wasted the chance for a week of direct negotiations, while the opposition was doing all it could.

"I think we are showing, through our presence, our behaviour, the number of documents we have submitted, the issues we are raising with the U.N., that we are here for a very constructive engagement with the United Nations, and that we have no partner so far to talk to," she said.

"They are physically not here."

The absence of the government delegation, led by chief negotiator Bashar al-Ja'afari, has effectively halted a peace process which has made little progress in eight rounds.

The latest round began last week with de Mistura hoping to discuss an agenda including constitutional and electoral reform.

But Ja'afari arrived a day late and left after two days, saying the opposition had "mined the road" to the talks by insisting that Assad could not play any interim role in Syria's political transition.

The delegation returned to Damascus to "consult and refresh", but Ja'afari initially threatened not to come back, which the opposition said would be "an embarrassment to Russia".

De Mistura said on Thursday that Ja'afari's delegation had confirmed it would return on Sunday, five days later than expected, for another stretch of talks lasting until Dec. 15. (Additional reporting by Marina Depetris; Editing by Angus MacSwan and Toby Chopra)